Groundwater flow and chemistry of the oases of Al Wahat, NE Libya

Abstract

The quality and geochemistry of groundwater are significantly affected by the depositional environment of aquifer sediments. Miocene sediments in Al Wahat area (Jalu, Awjilah and Shakherah Oases) in the Libyan Desert at the north-east of the country have been deposited in fluvial marginal marine and marine environments. The purposes of this paper are to describe the areal distribution of the dominant water quality constituents, to identify the major hydro-geochemical processes that affect the quality of water and to evaluate the relations of sedimentary depositional environments and groundwater flow to the quality and geochemistry of water in aquifer sediments of Post-Eocene. This study is the first investigation in Al Wahat Oasis and also in the whole Sahara, which introduces the importance of considering the end members and the synsedimentary influence for the interpretation of the aquifer hydrochemistry. The area involved in this study is within the boundaries 28°N–30°N and 21°E–23°E. Eighteen wells are selected in the area, including eight piezometers, and ten samples were analysed from wells used for domestic and agricultural purposes. Results show high and significant increase in total dissolved solids, especially Na+, Cl−, SO4 2− and NO3 − compared with the previous years. The chemical results for the groundwater samples in Al Wahat are classified according to the Stuyfzand groundwater classification system; the water type is mostly brackish and brackish-saline NaCl in the downstream direction and fresh-brackish NaHCO3 upstream. These water types indicate that groundwater chemistry is changed by cation exchange reactions during flushing of the diluted saline aquifer by freshwater from the south. The different stages of cation exchange produce a chromatographic sequence of groundwater types. These cation exchange reactions during the freshening process occur mainly in the intercalated clay, resulting in a Na+ increase, and peaks of K+ and Mg2+ in the aquifer. In the north, the synsedimentary marine influence on the groundwater is stronger and the abstraction for irrigation is higher. Upconing of deep saline water and anthropogenic pollution may contribute significantly to the aquifer water quality. Calcite equilibrium and gypsum dissolution are also important hydrochemical processes in the aquifer.